Electrical stimulation of electrically excitable tissue such as the brain and/or nerve tissue of the spinal cord or peripheral nerve can result in pain reduction and/or elimination for a living organism having the electrical stimulation performed. Thus, for example, medical leads having electrode contacts have been implanted near the spinal column of the human body to provide pain relief for chronic intractable pain. The nerve tissue within the spinal column is stimulated electrically to reduce pain sensations at other parts of the body.
Depending on the location of the pain sensation, and the particularities of each different human body, the parameters of the stimulation signals applied near the electrically excitable tissue are adjusted to optimize pain reduction or elimination. For example, the area of excitation within the spinal column and the intensity of excitation can be varied by corresponding adjustment of the parameters of the stimulation signals.
During acute trial stimulation, in order to vary the area of excitation, an array of electrodes may be implanted near the nerve tissue within the spinal column or peripheral nerve. Then, each of those electrodes can be configured to have a polarity such that the desired area of the nerve tissue within the spinal column is electrically stimulated. In addition, parameters of the respective stimulation signal applied on each of those implanted electrodes can be varied for a corresponding variation in the area of excitation within the spinal column and in the intensity of excitation at the pain site. Once the array of electrodes is implanted, a clinician who is knowledgeable of the effects of electrical stimulation may vary the parameters of the respective stimulation signal applied on each of the implanted electrodes. The patient may rate the effectiveness in pain reduction for each variation in the parameters of the stimulation signals. Then during chronic stimulation if electrical stimulation of nerve tissue does result in sufficient pain reduction for the patient, then the medical lead is implanted for the long term with stimulation signals having parameters that lead to optimized pain reduction for the particular patient.
Although spinal stimulation has proven effective for pain relief, there are problems associated with it, especially stimulation in the high cervical region. The conduit providing passage of the spinal cord in the lumbar vertebra provides more room for the spinal cord when compared to the conduit for the spinal cord in the cervical vertebra. In the lumbar region the spinal cord has a smaller diameter and therefore there is more room within the conduit of the lumbar vertebrae. As the spinal cord traverses up through the lumbar region of the spine to the cervical region, more and more peripheral nerves come into the spinal cord at the dorsal roots and therefore there is less room within. This poses a significant problem when placing the stimulation leads since the space in which to place the leads is diminished substantially.
Presently there are two basic styles of implantable leads available. One style is the percutaneously inserted lead, which is introduced through a Touhy needle. The implanting physician places the lead stimulating electrodes in an appropriate location using fluoroscopic visualization and the procedure is done under a local anesthetic. An example of this type of lead is disclosed in U.S. Pat. No. 4,379,462 issued to Borkan. Percutaneously inserted leads can be used for pain reduction/elimination, however, there are problems associated with these leads.
Percutaneously inserted leads are difficult to anchor and have a tendency to become dislodged. Even if the initial placement is accurate, lead migration can occur which can adversely affect paresthesia. Further, if the percutaneously inserted lead migrates enough to touch an incoming dorsal root, this can be very painful for the patient. Additionally, the area in which the patient is experiencing pain can move. This is a significant problem since percutaneous leads provide only limited means to change the area of stimulation if the lead migrates or if the needs of the patient change. Such means include reconfiguring the electrodes providing stimulation or performing additional surgery to adjust the lead's position. This problem could be resolved by enlarging the electrodes to cover more spinal cord area, however, the electrodes must be made small enough to fit through a Touhy needle. Thus, the stimulation area for percutaneous leads remains consequently small and because of this even a slight movement of the lead, especially laterally, can adversely affect paresthesia.
Another possible problem with percutaneous leads is their thickness is relatively large in comparison with the thickness of the dura mater in the high cervical region. Presently percutaneous leads are typically 0.050 inches in diameter. Because of the limited space in the high cervical region, if the lead is inserted improperly or if the lead migrates when placed in the cervical region where the dura mater is very thin, the percutaneous lead could possibly cause compression of the dura mater into the spinal cord causing discomfort, excess pain, and even paralysis.
The second basic spinal cord stimulation lead type is commonly referred to as a surgical lead and is surgically implanted in a procedure referred to as a laminotomy. An example of this type of lead is the RESUME® lead manufactured by Medtronic, Inc. of Minneapolis, Minn., the assignee of the present invention. This lead has four in-line electrodes located on a flat rectangular paddle at the distal end of the lead and the lead is normally implanted outside of the dura mater. Since leads of this type are surgically implanted, the size of the electrodes may be made larger than those of the percutaneously implanted leads. Further, various electrode combinations can be selected so that the area of stimulation may be moved along the midline of the spinal cord. The surgical lead can provide a broader stimulation pattern more suitable for midline and bilateral pain problems than the percutaneously inserted lead. Moreover, since it is surgically implanted it can be sutured to try and prevent dislodgement and reduce lead migration.
Surgical leads are less affected by the problem of lead migration because of the shape and size of the paddle and sutures may stabilize the lead. However, presently the paddles are made of silicon rubber, which requires a thickness of approximately 0.070 inches. Current technology does not allow the manufacture of desirably thin silicone rubber paddles suitable for locations with small extradural spaces such as the high cervical region due to production tolerances, coverage specifications, and internal anti-stretch components, which must be added to make the lead less elastic. Further, thin rubber coverage on silicon rubber paddles over internal components; such as the electrodes, are known to lack durability. Since the physician is trying to fit a rectangular lead into a cylindrical passage there is the potential for compression of the dura mater on the spinal cord. Therefore, inserting this rectangular lead still risks compression of the dura mater into the spinal cord causing discomfort, excess pain, and even paralysis.
Therefore, what is clearly needed is a method and apparatus for lead implantation in the high cervical region, which provides both improved fixation to prevent migration and an improved paddle structure to prevent compression of the dura mater against the spinal cord. What is further needed is a method for producing a thin paddle lead having improved fixation for implantation in the high cervical region.